Ultrasound color flow imaging is a widely used modality to enable the physician to view both venous and arterial blood flows. Many Doppler ultrasound systems employ linear array transducers which comprise a number of small, individual transducer elements arranged, side-by-side, in a single assembly. Two-dimensional images are produced in a sequential linear array scanner. By transmitting an ultrasound signal from each of the array elements (or group of elements) and receiving echo information, for each line of a final display (i.e., a B-mode display), the sequence of image formation results in a rectangular (or parallelogram) image format.
To create an image, a first group of elements is sequentially pulsed, and echoes from the ultrasound waveform are received, and enable derivation of the top B-mode line in the display. At the completion of the transmit/receive operation, a second group of elements is pulsed and the echoes received produce the second line of the display. This sequence continues until the last group of elements has been pulsed to produce the bottom or last line in the display. As a result of electronic scanning of these arrays, very high frame rates are possible which enable real time imaging of flow.
Prior art Doppler systems, which employ linear arrays, have included a user-operated control for altering the direction of transmission of the resultant array of beams. Such control mechanism alters all of the parallel beam paths in a like manner so that a region can be imaged. Prior art Doppler ultrasound systems have also employed a color assignment protocol to enable the user to rapidly identify a direction of arterial or venous flow.
Color map assignment in the prior art systems is based upon whether flow being imaged is away from the transducer array or towards the transducer array. If the transducer's beam path is oriented such that it is imaging flow in a direction away from the transducer (i.e., looking "downstream"), a first color assignment is made. By contrast, if the beam direction of the transducer looks "upstream" so that the flow is in a direction towards the transducer, a second color is assigned. One such color map function is termed "red-away, blue-towards" or RABT. A second color map procedure is "blue-away, red-towards" or BART.
Users of Doppler imaging systems generally prefer to have a single color assignment (e.g. red) to arterial flow and a single color assignment (e.g. blue) to venous flow. However, when a user changes the direction of transmission from a linear array, the ultrasound beam direction changes, for instance, from being directed in the upstream direction, to being directed in the downstream direction. More specifically, if a linear array transducer ultrasound beam is directed to image upstream arterial flow, a subsequent reorientation of the beam can cause it to pass a perpendicular drawn from the surface of the transducer, thus causing the imaging beam to look at downstream flow (with respect to the array). Assuming the ultrasound system is set to utilize a red-away, blue-towards color map, upon beam reorientation, the color of the arterial flow will change from blue to red. As known to those skilled in the art, a Doppler system determines the direction of flow by the sense of the change in the Doppler frequency.
However, it is known that users prefer to have arterial flow imaged in red, whether flowing away or towards the transducer, and venous flow imaged in blue, whether away or towards the transducer. As a result, some Doppler ultrasound systems have a user-operated control which enables reversal of the color map function. Thus, when the user perceives a color change resulting from a beam redirection, user-actuation of the mapping reversal control returns the flow colors to those which were in effect prior to the beam direction change.
Prior art Doppler displays also provide, at one side of the display, a color bar which indicates plural colors assigned to various flow rates, both towards and away from the transducer. Colors assigned to flow which is towards the transducer are often displayed at the top of the color bar and colors assigned to flow which is away from the transducer are displayed at the bottom of the color bar, with the two color areas separated by a "baseline". Such systems also include a control which enables more color intensities to be assigned for imaging of flow velocities in one direction, as compared to flow velocities in the other direction. This action, in effect, causes the baseline to alter its position along the color bar, either upwardly or downwardly, in dependence upon which flow is assigned the greater variety of velocity-representing colors.
When, however, the direction of transmission of the ultrasound beam is moved so that the blood flow being imaged transitions from an away direction to a towards direction, or vice versa, the colors on the top of the color bar switch to representing arterial flow and colors formerly representing venous flow are then on the bottom of the color bar. In addition, the baseline remains unchanged so that if it is off center, the range of colors which display the respective flows are reversed. Only when the user manually reverses the color map function and adjusts the baseline position does the color bar revert to its original color assignments. As a result, when the user perceives flow color changes, the user must be aware of the available means for change of flow color mapping to achieve flow color display continuity. As there are many controls on such ultrasound units, this adds a further level of complexity to the operation of the system.